The present invention relates to apparatus for making rows of perforations in webs which consist of paper, metallic foil, plastic foil or other sheet or strip material. More particularly, the invention relates to improvements in perforating tools which can be utilized in such apparatus. Still more particularly, the invention relates to improvements in blades which are utilized in perforating tools to make rows of slits, slots, holes or otherwise configurated perforations in running or stationary webs of paper or the like.
It is already known to assemble the perforating tool in an apparatus for perforating webs of paper or the like of a removable blade and a rotary holder for the blade. One longitudinal edge face of the blade has a row of perforating teeth in the form of triangular, square, rectangular or otherwise configurated projections which penetrate into a web once during each revolution of the holder. As a rule, the blade is made of sheet steel stock. The mounting of the blade in or on its holder is such that the teeth extend beyond the periphery of the holder so that they can penetrate into or otherwise deform or weaken a web which is caused to pass through the nip of the rotary holder with a suitable anvil, e.g., a counterroller against which the tips of the teeth abut when the one longitudinal edge face of the blade is caused to advance through the nip. The peripheral surface of the counterroller is normally hardened. When properly installed in the perforating apparatus, the holder for the blade and the counterroller are caused to bear against each other. The blade is disposed radially of the holder and its teeth bear against the periphery of the counterroller whenever the blade advances toward, past and beyond the aforementioned nip. The fact that the holder and the counterroller bear against each other entails at last some flexing of such components.
It has been found that, when a perforating apparatus of the above outlined character is in actual use, the extent of flexing of the holder and/or of the counterroller decreases with increasing RPM of the holder. This, in turn, entails a more pronounced and more rapid wear upon those perforating teeth which are located substantially midway between the axial ends of the holder, i.e., midway between the transverse edge faces of the blade. In other words, while the teeth which are adjacent to the two axial ends of the holder are still of acceptable size and shape, the size and/or shape of teeth in the region of the center of the holder deviates considerably from the original size and/or shape. Consequently, when the RPM of the holder is reduced and the flexural stresses upon the holder and the counterroller increase, the centrally located perforating teeth remain out of contact with the web or fail to penetrate all the way through the web so that the rows of perforations are interrupted or the perforations at the center of each row are less pronounced that those in the regions of the two marginal portions of the web. Thus, while the end portions of perforated zones of the web offer a relatively low resistance to tearing or flexing, the median portions of such perforated zones offer a much more pronounced resistance to deformation or breakage because the perforations in the median portions are less pronounced or are absent in their entirety. This can create numerous problems during further processing of the web.